1. Field of the Invention
The present invention relates to a method of manufacturing electrode foil for aluminium electrolytic capacitors, and more specifically, to an etching technology for anode aluminium foil used for medium and high voltage capacitors.
2. Description of the Related Art
Recently, as electronic appliance sets are miniaturized and their reliability are increased, the requirements for miniaturization and reduced costs of users for aluminium electrolytic capacitors are rapidly increasing. Thus, a capacitance per unit area of electrode foil for aluminium electrolytic capacitors (hereinafter, referred to as electrode foil) must be increased as compared with conventional ones.
A conventional method of manufacturing electrode foil will be described below.
Electrode foil is composed of aluminium foil whose effective surface area is increased by electrochemically or chemically etching the aluminium foil for miniaturizing capacitors. Although various etching methods have been examined for increasing the surface area, it is generally achieved in such a manner that the aluminium foil is continuously put into several kinds of different etching baths and applied with a current or chemically dissolved in the respective baths to gradually increase the surface area of the aluminium foil and then finally washed. Especially, in case of the electrode foil used for medium and high voltage capacitors, etched pits (hereinafter, referred to as pits) formed in a preceding stage must be increased to a diameter suitable for a voltage in use in a succeeding stage and how to effectively increase pit diameters is an important consideration.
Conventionally, pits are produced by electric etching in an acid solution containing chlorine ions in the preceding stage and the diameter of the pits is increased by chemical dissolution in an acid solution of high temperature in the succeeding stage. This method which relies on the chemical dissolution in the succeeding stage is liable to be affected by a difference (dispersion) of the dissolving property among the lots of raw foil and the diameter of pits cannot be uniformly increased. Thus, it is difficult to increase a capacitance of the electrode foil. With respect to this point, as disclosed in Japanese Patent Unexamined Publication No. Hei 2-303018, Japanese Patent Unexamined Publication No. Hei 3-101213 and Japanese Patent Unexamined Publication No. Hei 3-136226, a technology for carrying out electrical etching process by use of an aqueous solution of nitric acid in the succeeding stage is known.
Nevertheless, the above known technology is not satisfactory in the point that the diameter of pits is effectively increased while an amount of dissolved aluminium is controlled because there is an insufficient point with respect to the constitution and liquid temperature of the aqueous solution of nitric acid.